A hard wear resistant insert for use in a center vacuum rotary drill bit is typically formed of cemented tungsten carbide and is secured by brazing the insert within a slot formed within a top working surface of the body of the rotary drill bit. The hard wear resistant insert secured within the top working surface of the rotary drill bit body typically wears or dulls before the drill bit body such that the user of the center vacuum rotary drill bit must either replace the entire rotary drill bit, attempt to replace the insert with a new insert of similar size, or attempt to resharpen the insert. Replacement of the entire center vacuum rotary drill bit is costly and resharpening of the insert may only be performed a limited number of times before the top working surface of the drill bit body frictionally engages the material to be cut. Inevitably, replacement of the worn hard wear resistant insert with a new hard wear resistant insert is the selected approach.
It will be appreciated that after prolonged use both the hard wear resistant insert of the rotary drill bit body and the top working surface of the bit body wear such that the depth of the slot within the center vacuum rotary drill bit body changes.
Because the height of the hard wear resistant insert and/or depth of the slot within the center vacuum rotary drill bit body are typically unique to a given manufacturer, substitution of one size insert among various center vacuum rotary drill bit bodies is difficult if not impossible. Substitution of worn inserts with comparably sized inserts is important because a dissimilar insert may not clear the top working surface of the center vacuum rotary drill bit body or the new insert may project so far above the center vacuum rotary drill bit body that the insert is susceptible to fracturing during use.
Exemplary of this problem is a comparison of a HKCV3 13/8 inch center vacuum rotary drill bit obtainable from Kennametal Inc., and a comparable 13/8 inch center vacuum rotary drill bit obtainable from American Mine Tool. The HKCV3 13/8 inch center vacuum rotary drill bit includes an insert approximately 0.67 inches in height within a 0.55 inch deep slot formed within the top working surface of the rotary drill bit. The comparable 13/8 inch center vacuum rotary drill bit obtainable from American Mine Tool includes an insert of approximately 0.74 inches in height within an approximately 0.62 inch deep slot. It will be appreciated that the Kennametal insert cannot be simply substituted within the American Mine Tool slot because the slots are of different depths, thereby effecting the amount of insert exposed during drilling, i.e., clearance, and inevitably the performance of the drill bit.
One possible solution is to manufacture a variety of new inserts having acceptable insert heights for the corresponding slot depths formed within the center vacuum rotary drill bit bodies. The separate manufacture of new inserts for use with a specific center vacuum rotary drill bit body all but eliminates any savings which may be realized in substituting a readily available insert within another manufacturer's center vacuum rotary drill bit body because of the variety of inserts which must be inventoried to meet industry requirements.
In view of the foregoing, there is a significant need for an insert spacer assembly to allow for the use of various insert designs within a variety of rotary drill bit bodies to accommodate dissimilar slot depths and insert heights. In addition, there is a significant need for an insert spacer assembly to allow for the use of inserts made of less material than comparable inserts typically employed in a similar slot, thereby resulting in substantial cost savings.
To alleviate the aforementioned problems, we have invented a novel insert spacer assembly for adjusting the elevation of a hard, wear resistant insert within a slot formed within a top working surface of a rotary drill bit body. The insert spacer assembly includes a shim and a spacer bar. The coefficient of expansion of the spacer bar and the shim are each greater than or equal to 10 micro in./in./degree C. to effectively distribute the force of drilling over the slot and reduce the stress load on the insert. It is believed that if the coefficient of expansion of the spacer bar is less than 10 micro in./in./degree C. then the spacer bar will be susceptible to internal stress fracture during brazing of the insert and heat treatment of the bit body.
The combination of the shim and spacer bar elevates the insert within the slot with respect to the top working surface of the rotary drill bit body such that most any style and height of insert may be used within any rotary drill bit body. It will be appreciated that because most any height of insert may now be used in any depth of slot and provide substantially the same rotary drill bit effectiveness, substantial savings may also be realized in using less material to form the insert.